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Fusion Essay, Research Paper

Fusion reactions
are inhibited by the electrical repulsive force that acts between two
positively charged nuclei. For fusion to occur, the two nuclei must
approach each other at high speed to overcome the electrical
repulsion and attain a sufficiently small separation (less than
one-trillionth of a centimeter) that the short-range strong nuclear
force dominates. For the production of useful amounts of energy, a
large number of nuclei must under go fusion: that is to say, a gas of
fusing nuclei must be produced. In a gas at extremely high
temperature, the average nucleus contains sufficient kinetic energy
to undergo fusion. Such a medium can be produced by heating an
ordinary gas of neutral atoms beyond the temperature at which
electrons are knocked out of the atoms. The result is an ionized gas
consisting of free negative electrons and positive nuclei. This gas
constitutes a plasma. Plasma, in physics, is an electrically
conducting medium in which there are roughly equal numbers of
positively and negatively charged particles, produced when the atoms
in a gas become ionized. It is sometimes referred to as the fourth
state of matter, distinct from the solid, liquid, and gaseous states.
When energy is continuously applied to a solid, it first melts, then
it vaporizes, and finally electrons are removed from some of the
neutral gas atoms and molecules to yield a mixture of positively
charged ions and negatively charged electrons, while overall neutral
charge density is maintained. When a significant portion of the gas
has been ionized, its properties will be altered so substantially
that little resemblance to solids, liquids, and gases remains. A
plasma is unique in the way in which it interacts with itself with
electric and magnetic fields, and with its environment. A plasma can
be thought of as a collection of ions, electrons, neutral atoms and
molecules, an photons in which some atoms are being ionized
simultaneously with other electrons recombining with ions to form
neutral particles, while photons are continuously being produced and
absorbed. Scientists have estimated that more than 99 percent of the
matter in the universe exists in the plasma state. All of the
observed stars, including the Sun, consist of plasma, as do
interstellar and interplanetary media and the outer atmospheres of
the planets. Although most terrestrial matter exists in a solid,
liquid or gaseous state, plasma is found in lightning bolts and
auroras, in gaseous discharge lamps (neon lights), and in the crystal
structure of metallic solids. Plasmas are currently being studied as
an affordable source of clean electric power from thermonuclear
fusion reactions. The scientific problem for fusion is thus the
problem of producing and confining a hot, dense plasma. The core of a
fusion reactor would consist of burning plasma. Fusion would occur
between the nuclei, with electrons present only to maintain
macroscopic charge neutrality. Stars, including the Sun, consist of
plasma that generates energy by fusion reactions. In these ?natural
fusion reactors? the reacting, or burning, plasma is confirmed by its
own gravity. It is not possible to assemble on Earth a plasma
sufficiently massive to be gravitationally confined. The hydrogen
bomb is an example of fusion reactions produced in an uncontrolled,
unconfined manner in which the energy density is so high that the
energy release is explosive. By contrast, the use of fusion for
peaceful energy generating requires control and confinement of a
plasma at high temperature and is often called controlled
thermonuclear fusion. In the development of fusion power technology,
demonstration of ? energy breakeven? is taken to signify the
scientific feasibility of fusion. At breakeven, the fusion power
produced by a plasma is equal to the power input to maintain the
plasma. This requires a plasma that is hot, dense, and well confined.
The temperature required, about 100 million Kelvins, is several times
that of the Sun. The product of the density and energy confinement
time of the plasma (the time it takes the plasma to lose its energy
if not replaced) must exceed a critical value. There are two main
approaches to controlled fusion ? namely, magnetic confinement and
inertial confinement. Magnetic confinement of plasmas is the most
highly developed approach to controlled fusion. The hot plasma is
contained by magnetic forces exerted on the charged particles. A
large part of the problem of fusion has been the attainment of
magnetic field configurations that effectively confine the plasma. A
successful configuration must meet three criteria: (1) the plasma
must be in a time-independent equilibrium state, (2) the equilibrium
must be macroscopically stable, and (3) the leakage of plasma energy
to the bounding wall must be small. A single charged particle tends
to spiral about a magnetic line of force. It is necessary that the
single particle trajectories do not intersect the wall. Moreover, the
pressure force, arising from the thermal energy of all the particles,
is in a direction to expand the plasma. For the plasma to be in
equilibrium, the magnetic force acting on the electric current within
the plasma must balance the pressure force at every point in the
plasma. The equilibrium thus obtained has to be stable. A plasma is
stable if after a small perturbation it returns to its original
state. A plasma is continually perturbed by random thermal “noise”
fluctuations. If unstable, it might depart from its equilibrium state
and rapidly escape the confines of the magnetic field (perhaps in
less than one-thousandth of a second). A plasma in stable equilibrium
can be maintained indefinitely if the leakage of energy from the
plasma is balanced by energy input. If the plasma energy loss is too
large, then ignition cannot be achieved. An unavoidable diffusion of
energy across the magnetic field lines will occur from the collisions
between the particles. The net effect is to transport energy from the
hot core to the wall. This transport process, known as classical
diffusion, is theoretically not strong in hot fusion plasmas and is
easily compensated for by heat from the alpha particle fusion
products. In experiments, however, energy is lost from plasma more
rapidly than would be expected from classical diffusion. The observed
energy loss typically exceeds the classical value by a factor of
10-100. Reduction of this anomalous transport is important to the
engineering feasibility of fusion. An understanding of anomalous
transport in plasmas in terms of physics is not yet in hand. A
viewpoint under investigation is that the anomalous loss is caused by
fine-scale turbulence in the plasma. However, turbulently fluctuating
electric and magnetic fields can push particles across the confining
magnetic field. Solution of the anomalous transport problem involves
research into fundamental topics in plasma physics, such as plasma
turbulence. Many different types of magnetic configurations for
plasma confinement have been devised and tested over the years. This
has resulted in a family of related magnetic configurations, which
may be grouped into two classes: closed, toroidal configurations and
open, linear configurations. Toroidal devices are the most highly
developed. In a simple straight magnetic field the plasma would be
free to stream out the ends. End loss can be eliminated by forming
the plasma and field in the closed shape of a doughnut, or torus, or,
in an approach called mirror confinement, by “plugging” the ends
of such a device magnetically and electrostatically. In the inertial
confinement a fuel mass is compressed rapidly to densities 1,000
to10,000 times greater than normal by generating a pressure as high
as 1017 pascals for periods as short as nanoseconds. Near the end of
this time period the implosion speed exceeds about 300,000 meters per
second. At maximum compression of the fuel, which is now in a cool
plasma state, the energy in converging shock waves is sufficient to
heat the vary center of the fuel to temperatures high enough to
induce fusion reactions. If the product of mass and size of this
highly compressed fuel material is large enough, energy will be
generated through fusion reactions before the plasma disassembles.
Under proper conditions, more energy can be released than is required
to compresses, and shock-heat the fuel to thermonuclear burning
conditions. The physical processes in ICF bear relationship to those
in thermonuclear weapons and in star formation?namely, gravitational
collapse, compression heating, and the onset of nuclear fusion. The
situation in star formation differs in one respect: after
gravitational collapse ceases and star begins to expand again due to
heat from exoergic nuclear fusion reactions, the expansion is
arrested by the gravity force associated with the enormous mass of
the star. In a star a state of equilibrium in both size and
temperature is achieved. In ICF, by contrast, complete disassembly of
fuel occurs. The fusion reaction least difficult to achieve combines
a deuteron (the nucleus of the deuterium atom) with a triton (the
nucleus of a tritium atom). Both nuclei are isotopes of the hydrogen
nucleus and contain a single unit of positive electric charge.
Deuterium-tritium (D-T) fusion requires the nuclei to have lower
kinetic energy than is needed for the fusion of more highly charged
heavier nuclei. The two products of the reaction are an alpha
particle (nucleus of the helium atom) at an energy of 3.5 million
electron volts (MeV) and a neuron at an energy of 14.1 MeV. (One MeV
is the energy equivalent of 10 billion Kelvin.). The neutrons,
lacking electric charge, is not affected by electric or magnetic
fields within the plasma and can escape the plasma to deposit its
energy in a material, such as lithium, which can surround the plasma.
The electrically charge alpha particle collides with the deuterons
and tritons (by their electrical interaction) and can be magnetically
confined within the plasma. It there by transfers its energy to the
reacting nuclei. When this redeposition of the fusion energy into the
plasma exceeds the power lost from the plasma (by electromagnetic
radiation, conduction, and convection), the plasma will be
self-sustaining, or ?ignited.? With deuterium and tritium as the
fuel, the fusion reactor would be an effectively inexhaustible source
of energy. Deuterium is obtained from seawater. About one in every
3,000 water molecules contains a deuterium atom. There is enough
deuterium in the oceans to provide for the world?s energy needs for
billions of years. One gram of fusion fuel can produce as much energy
as 9,000 liters of oil. The amount of deuterium found naturally in
one liter of water is the energy equivalent of 300 liters of
gasoline. Tritium is bred in the fusion reactor. It is generated in
the lithium blanket as a product of the reactor in which neutrons are
captured by the lithium nuclei. A fusion reactor would have several
attractive safety features. First, it is not subject to a runaway, or
“meltdown,” accident as is a fission reactor. The fusion reaction
is not a chain reaction; it requires a hot plasma. Accidental
interruption of a plasma control system would extinguish the plasma
and terminate fusion. Second, the products of a fusion reaction are
not radioactive; hence, no long-term radioactive wastes would be
generated. Neutron bombardment would activate the walls of the
containment vessel, but such activated material is shorter-lived and
less toxic than the waste products of a fission reactor. Moreover,
even this activation problem may be eliminated, either by the
development of advanced, low-activation materials, such as
vanadium-based materials, or by the employment of “advanced”
fusion-fuel cycles that do not produce neutrons, such as the fusion
of deuterons with helium-3 nuclei. Nearly neutron-free fusion
systems, which require higher temperatures than D-T fusion, might
make up a “second generation” of fusion reactors). Finally, a
fusion reactor would not release the gaseous pollutants that
accompany the combustion of fossil fuels; hence, fusion would not
produce a greenhouse effect. The fusion process has been studied as
part of nuclear physics for much of the 20th century. In the late
1930s the German-born physicist Hans A. Bethe first recognized that
the fusion of hydrogen nuclei to form deuterium is exoergic (there is
release of energy) and, together with subsequent reactions, accounts
for the energy source in stars. Work proceeded over the next two
decades, motivated by the need to understand nuclear matter and
forces, to learn more about the nuclear physics of stellar objects,
and to develop thermonuclear weapons (the hydrogen bomb) and predict
their performance. During the late 1940s and early 1950s, research
programs in the United States, United Kingdom, and Soviet Union began
to yield a better understanding of nuclear fusion, and investigators
embarked on ways of exploiting the process for practical energy
production. This work focused on the use of magnetic fields and
electromagnetic forces to contain extremely hot gases called plasmas.
A plasma consists of unbound electrons and positive ions whose motion
is dominated by electromagnetic interactions. It is the only state of
matter in which thermonuclear reactions can occur in a
self-sustaining manner. Astrophysics and magnetic fusion research,
among other fields, require extensive knowledge of how gases behave
in the plasma state. The inadequacy of the then-existent knowledge
became clearly apparent in the 1950s as the behavior of plasma in
many of the early magnetic confinement systems proved too complex to
understand. Moreover, researchers found that confining fusion plasma
in a “magnetic trap” was far more challenging than they had
anticipated. Plasma must be heated to tens of millions of degrees
Kelvin or higher to induce and sustain the thermonuclear reaction
required to produce usable amounts of energy. At temperatures this
high, the nuclei in the plasma move rapidly enough to overcome their
mutual repulsion and fuse. It is exceedingly difficult to contain
plasmas at such a temperature level because the hot gases tend to
expand and escape from the enclosing structure. The work of the major
American, British, and Soviet fusion programs was strictly classified
until 1958. That year, research objectives were made public, and many
of the topics being studied were found to be similar, as were the
problems encountered. Since that time, investigators have continued
to study and measure fusion reactions between the lighter elements
and have arrived at more accurate determinations of reaction rates.
Also, the formulas developed by nuclear physicists for predicting the
rate of fusion-energy generation have been adopted by astrophysicists
to derive new information about the structure of the stellar interior
and about the evolution of stars. The late 1960s witnessed a major
advance in efforts to harness fusion reactions for practical energy
production: the Soviets announced the achievement of high plasma
temperature (about 3,000,000 K), along with other physical
parameters, in a tokamak, a toroidal magnetic confinement system in
which the plasma is kept generally stable both by an externally
generated, doughnut-shaped magnetic field and by electric currents
flowing within the plasma itself. (The basic concept of the tokamak
had been first proposed by Andrey D. Sakharov and Igor Y. Tamm around
1950.) Since its development, the tokamak has been the focus of most
research, though other approaches have been pursued as well.
Employing the tokamak concept, physicists have attained conditions in
plasmas that approach those required for practical fusion-power
generation. Work on another major approach to fusion energy, called
inertial confinement fusion (ICF), has been carried on since the
early 1960s. Initial efforts were undertaken in 1961 with a
then-classified proposal that large pulses of laser energy could be
used to implode and shock-heat matter to temperatures at which
nuclear fusion would be vigorous. Aspects of inertial confinement
fusion were declassified in the 1970s, but a key element of the
work–specifically the design of targets containing pellets of
fusion fuels–still is largely secret. Very painstaking work to
design and develop suitable targets continues today. At the same
time, significant progress has been made in developing high-energy,
short-pulse drivers with which to implode millimeter-radius targets.
The drivers include both high-power lasers and particle accelerators
capable of producing beams of high-energy electrons or ions. Lasers
that produce more than 100,000 joules in pulses on the order of one
nanosecond (10-9 second) have been developed, and the power available
in short bursts exceeds 1014 watts. Best estimates are that practical
inertial confinement for fusion energy will require either laser or
particle-beam drivers with an energy of 5,000,000 to 10,000,000
joules capable of delivering more than 1014 watts of power to a small
target of deuterium and tritium .

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